1. Field of the Invention
The present invention relates to a multi-layered plain bearing which comprises a steel back and an aluminum-base bearing alloy layer which is bonded to the steel back via an intermediate layer made of an aluminum alloy, and to a producing method thereof.
2. Brief Description of the Art
Aluminum alloy plain bearings have excellent properties of conformability and wear resistance, and have been broadly used for high power engines of motor vehicles and general industrial machines. In general, the aluminum alloy plain bearings have a three-layer structure in which an aluminum bearing alloy layer is bonded to a steel back via an intermediate layer.
The intermediate layer has been conventionally made of a comparatively soft material of pure aluminum or an aluminum alloy. However, coping with the recent trend of high power engines, aluminum bearing alloys tend to be required to have further improved fatigue and wear resistance. A solution of such a requirement has been to strengthen the aluminum bearing alloys by adding alloying elements of Cu, Zn, Mg, Si and so on. With such strengthening of the aluminum bearing alloys, the intermediate layer has been also tried to have improved fatigue resistance by adding strengthening elements including Mn.
The aluminum bearing alloy with strengthening additive elements of Cu, Zn, Mg, Si and so on is so produced that an aluminum bearing alloy layer is bonded to a steel back via an intermediate aluminum alloy layer by rolling and subsequently the layered structure is subjected to a solid solution-treatment of heating to a temperature of not lower than 400xc2x0 C. followed by rapid cooling. However, when it is heated to the solid solution treatment temperature of not lower than 400xc2x0 C., an inter-metallic compound of Alxe2x80x94Fe is generated at the interface between the steel back and the intermediate aluminum alloy layer. Since the inter-metallic compound of Alxe2x80x94Fe is quite brittle, there is a risk that the intermediate aluminum alloy layer might be delaminated from the steel back under operating conditions in which engine bearings are subjected to a varying load.
According to JP-A-61-272358, in order to avoid occurrence of brittle inter-metallic compounds at an interface between a steel back and an intermediate layer, heating to a solid solution treatment temperature is performed at a higher heating rate and the aimed temperature is kept for fully short time so as not to rise therefrom once after attaining the temperature. However, such a process is not practical because it requires a strict temperature control.
On the other hand, it is necessary to perform the solid solution treatment at a temperature as high as possible in order to improve the strength of the aluminum bearing alloys. For this, an intermediate aluminum alloy layer is desired.
Accordingly, an object of the present invention is to prevent occurrence of brittle inter-metallic compounds at the interface between the steel back and the intermediate aluminum alloy layer whereby realizing a high temperature in a solid solution treatment and to improve aluminum bearing alloy in strength so that a multi-layered plain bearing having excellent fatigue resistance and a producing method thereof are provided.
According to one aspect of the present invention, there is provided a multi-layered plain bearing which comprises a steel back, an intermediate layer made of an aluminum alloy and an aluminum-base bearing alloy layer comprising one or more elements selected from the group consisting of Cu, Zn, Mg and Si, the aluminum-base bearing alloy layer being bonded to the steel back via the intermediate layer and subsequently subjected to a solid solution treatment at a temperature of not lower than 400xc2x0 C., wherein the adjacent region of the intermediate layer to the steel back consists of, by mass, 2% to 8% of Si, and the balance of Al and incidental impurities.
When producing the multi-layered plain bearing, the layered material is heated to a temperature of not lower than 400xc2x0 C. in the solid solution treatment, the element(s) of Cu, Zn, Mg and/or Si dissolves in the aluminum matrix so that the aluminum-base bearing alloy is hardened and strengthened by rapid cooling after the above heating whereby improving the multi-layered plain bearing in fatigue resistance. During the solid solution treatment, while an Alxe2x80x94Fexe2x80x94Si inter-metallic compound precipitates preferentially rather than the Alxe2x80x94Fe inter-metallic compound at the adjacent side of the aluminum alloy intermediate layer to the steel back, the Alxe2x80x94Fexe2x80x94Si inter-metallic compound does not precipitate at a temperature only above 400xc2x0 C. until above 550xc2x0 C. Thus, it is possible to effectively prevent occurrence of the brittle Alxe2x80x94Fe inter-metallic compound to perform the solid solution treatment.
According to another aspect of the present invention, there is provided a multi-layered plain bearing which comprises a steel back, an intermediate layer made of an aluminum alloy and an aluminum-base bearing alloy layer comprising one or more elements selected from the group consisting of Cu, Zn, Mg and Si, the aluminum-base bearing alloy layer being bonded to the steel back via the intermediate layer and subsequently subjected to a solid solution treatment at a temperature of not lower than 400xc2x0 C. The intermediate layer comprises a sub-layer adjacent to the steel back, and at least one sub-layer other than the sub-layer adjacent to the steel back, wherein the sub-layer adjacent to the steel back consists of, by mass, 2% to 8% of Si, and the balance of Al and incidental impurities, and the at least one sub-layer other than the sub-layer adjacent to the steel back consists of, by mass, at least one element selected from the group consisting of from more than 0% to not more than 2% of Mn, from more than 0% to not more than 2% of Cu, from more than 0% to not more than 2% of Mg, from more than 0% to not more than 2% of Fe, and the balance of Al and incidental impurities.
Also in this case, as described in the first case under the first aspect, when the multi-layered plain bearing is heated to a temperature of not lower than 400xc2x0 C. during a solid solution treatment, it is possible to effectively prevent occurrence of the brittle Alxe2x80x94Fe inter-metallic compound to increase the bonding strength between the intermediate layer and the steel back. During the solid solution treatment, in the at least one sub-layer other than the sub-layer adjacent to the steel back (i.e. at least one sub-layer of the intermediate layer existing at the side of aluminum-base bearing alloy layer), the element(s) of Cu, Zn, Mg and/or Si dissolves in the aluminum matrix so that the aluminum-base bearing alloy is hardened and strengthened by rapid cooling after the above heating whereby improving the multi-layered plain bearing in fatigue resistance.
The intermediate layer can have two functions or advantages which are an effect of preventing occurrence of the Alxe2x80x94Fe inter-metallic compound during the solid solution treatment and a strengthening effect by composing it with the plurality of sub-layers while strengthening the aluminum-base bearing alloy layer by the solid solution treatment, whereby it is possible to provide the multi-layered plain bearing with excellent fatigue resistance.
Here, grounds of the criticality will be described with regard to the additive alloying elements in the intermediate aluminum alloy layer.
(1) Si (2 to 8 mass %)
Si dissolves in the aluminum matrix and crystallizes as highly hard Si particles to increase the alloy hardness. When the multi-layered material is heated to a temperature of not lower than 400xc2x0 C. in the solid solution treatment, the Alxe2x80x94Fe inter-metallic compound is not precipitated in the intermediate layer due to presence of Si. If the Si amount is less than 2 mass %, the above effects can not be attained. If the Si amount exceeds 8 mass %, plastic workability such as ductility is markedly deteriorated. Preferably the Si amount is 6 to 8 mass %.
(2) Mn or Mg (from More than 0 to not More than 2 mass %)
Mn or Mg dissolves in the aluminum matrix or crystallizes as an inter-metallic compound to increase the alloy strength. If its amount exceeds 2 mass %, the alloy becomes too hard thereby deteriorating the plastic workability of the alloy. Preferably the Mn or Mg amount is 0.7 to 1.5 mass %.
(3) Cu (from More Than 0% to Not More Than 2 mass %)
Cu dissolves in the aluminum matrix to increase the alloy strength including fatigue strength. If the Cu amount exceeds 2 mass %, the alloy becomes too hard thereby deteriorating the plastic workability of the alloy. Preferably the Cu amount is 0.7 to 1.7 mass %.
(4) Fe (from More than 0% to not More than 2 mass %)
Fe dissolves in the aluminum matrix or crystallizes to disperse as an inter-metallic compound thereby remarkably increasing the alloy strength. If the Fe amount exceeds 2 mass %, crystallizing iron inter-metallic compounds increase to markedly deteriorate plastic workability of the alloy. Preferably the Fe amount is 0.07 to 1 mass %.
The aluminum-base bearing alloy preferably comprises one or more elements selected from the following element groups (1) to (5).
(1) 3 to 20 mass % of Sn
Sn improves the surface performance of the bearing, which includes anti-seizure property, conformability and embeddability. If the Sn amount is less than 3 mass %, such an improvement effect can not be attained. If the Sn amount exceeds 20 mass %, the aluminum-base bearing alloy is deteriorated in mechanical property to become undurable under severe conditions such as in high power engines. Preferably, the Sn amount is 6 to 15 mass %.
(2) At Least One Element Selected from the Group Consisting of Cu, Zn, Mg and Si in an Amount or a Total Amount of 0.1 to 7 mass %.
These optional elements improve the strength of the aluminum matrix and can be forcedly dissolved by subjecting the aluminum-base bearing alloy to the solid solution treatment. It is also possible to precipitate fine inter-metallic compounds from the matrix by rapid cooling in the solid solution treatment. Si dissolves in the aluminum matrix. Crystallized elemental silicon is microscopically dispersed in the alloy to improve the alloy in fatigue strength and contribute to improvements of anti-seizure property and wear resistance. If the amount(s) of one or more of the elements is less than 0.1 mass %, such effects or advantages can not be attained. If the amount(s) of one or more of the elements exceeds 7 mass %, the precipitated inter-metallic compounds become coarse. The amount(s) of one or more of the elements is preferably of 0.5 to 6 mass % in total.
(3) At Least One Element Selected from the Group Consisting of Mn, V, Mo, Cr, Co, Fe, Ni and W in an Amount or a Total Amount of 0.01 to 3 mass %.
These optional elements dissolves in the aluminum matrix or crystallizes elementally or as inter-metallic compounds to improve the strength of the alloy. If the amount(s) of one or more of the elements is less than 0.01 mass %, such an effect is not expectable. If the amount(s) of one or more of the elements exceeds 3 mass %, the inter-metallic compounds become too coarse whereby deteriorating physical properties of the bearing alloy and plastic workability such as when rolling. The amount(s) of one or more of the elements is preferably 0.2 to 2 mass % in total.
(4) At Least One Element Selected from the Group Consisting of B (boron), Ti and Zr in an Amount or a Total Amount of 0.01 to 2 mass %.
These optional elements dissolve in the aluminum matrix to improve the fatigue strength of the bearing alloy. If the amount(s) of one or more of the elements is less than 0.01 mass %, such an effect is not expectable. If the amount(s) of one or more of the elements exceeds 2 mass %, the inter-metallic compounds become brittle. The amount(s) of one or more of the elements is preferably 0.02 to 0.5 mass % in total.
(5) At Least One Element Selected from the Group Consisting of Pb, Bi and In in an Amount or a Total Amount of not More than 3 mass %.
These additive elements improve machinability and anti-seizure property of the bearing alloy. If the amount(s) of one or more of the elements exceeds 3 mass %, it becomes difficult to uniformly distribute the element(s) in the aluminum matrix and the bearing alloy is deteriorated in strength.
The multi-layered plain bearing of the invention is produced by the process comprising the following steps:
bonding the intermediate layer of aluminum alloy to the aluminum-base bearing alloy layer;
bonding the aluminum-base bearing alloy layer to the steel back via the intermediate layer of aluminum alloy; and
subjecting the thus obtained multi-layered material to a solid solution treatment of heating to a temperature of not lower than 400xc2x0 C. followed by a rapid cooling to strengthen the aluminum-base bearing alloy layer.